1. Field of the Invention
The present invention relates to an Orthogonal Frequency Division Multiplexing (OFDM) receiver, an OFDM reception method and a terrestrial digital receiver. Particularly, it relates to an OFDM receiver, an OFDM reception method and a terrestrial digital receiver applying antenna diversity.
2. Description of the Related Art
The terrestrial digital broadcasting employs a modulation method of Orthogonal Frequency Division Multiplexing (OFDM). OFDM is one form of the multi-carrier methods, that is, the modulation methods of transmitting information with a lot of carriers. Accordingly, it is less subject to the transmission line (especially, the multipath) compared to the single carrier method. In addition, the OFDM has a buffering period called a guard interval in the head portion of a single unit of the transmission information. Also in this respect, OFDM is considered multipath resistant.
However, the functions (the multi-carrier and the guard interval) that OFDM has by nature are not sufficient for a mobile-type OFDM receiver which is likely to be used under a severe environment such as in a vehicle that is traveling at a high speed. Because of this, other countermeasure technology against multipath, typically, antenna diversity, is used in combination.
As an example of an OFDM receiver applying the antenna diversity, an art (hereinafter denoted as a “conventional prior art”) disclosed in Japanese Laid-Open Patent Publication No. 2003-229830 is known. The conventional prior art receives an OFDM signal with a plurality of antennas, determines a correlation value between a down-converted OFDM signal in the IF bands for each antenna and a delay OFDM signal which is delayed from the OFDM signal by the amount equivalent to a single effective symbol, calculates a Carrier to Noise ratio (C/N ratio) (that is, the ratio of the addition noise electricity to the signal electricity at the reception point) based on the correlation value, selects an equalization carrier signal with the highest C/N ratio among carrier signals of the same number that are acquired from each branch circuit, and decodes it.
As mentioned above, the conventional prior art “determines a correlation value between each down-converted OFDM signal in the IF band for each antenna and a delay OFDM signal which is delayed from the OFDM signal by the amount corresponding to a single effective symbol, calculates a C/N ratio Carrier to Noise ratio (C/N ratio)” based on the correlation value, which is, put briefly, interpreted as “using the information of the guard interval.
FIG. 8A is a conceptual drawing of a guard interval. Three consecutive three symbols (K−1, K, K+1) in terms of time are now assumed as shown in FIG. 8A. K denotes a current symbol, K−1 denotes a previous symbol in terms of time, and K+1 denotes a subsequent symbol K in terms of time. Lengths of individual symbol periods T are identical, and, for example, the length of the symbol period T of the terrestrial digital broadcasting is 1.008 μs (in the case of Mode 3). Every symbol is comprised of an effective symbol period Tu in which transmission information is housed and a guard interval period Tg (hatched portion) with a constant length which has been added to a head portion of the effective symbol period Tu. Information of some (the end portion) of the effective symbol period Tu subsequent to the guard interval period Tg is copied to the guard interval period Tg of each symbol.
FIG. 8B is a conceptual drawing on use of the guard interval information in the conventional prior art. In FIG. 8B, a non-delay symbol and a delay symbol are the identical symbol (for example, Symbol K). The delay symbol is a symbol delayed from Symbol K by a predetermined period of time (T−Tg). The delay symbol is equivalent to the “delay OFDM signal” in the conventional prior art. Next, when correlation of the two symbols (non-delay symbol and delay symbol) is evaluated, the evaluated value is large at a superimposed period of the end portion of the effective symbol of the non-delay symbol and the guard interval period Tg of the delay symbol (that is, the superimposed period of Y and Z). This is because the information of Y and Z is originally identical.
The conventional prior art as mentioned above can be interpreted as an art that evaluates the correlation on Y and Z based on the principle, and selects an equalization carrier signal with the highest C/N ratio and decodes it.
However, the following two points are pointed out for the conventional prior art. First, diversity of the conventional prior art is equivalent to a so-called composition diversity that selects the signal with the best C/N ratio among the signals that have been received with a plurality of antennas. Although the composition diversity is superior in performance to the method of switching a plurality of antennas (i.e., selection diversity), it has a drawback of increase in hardware scale. Accordingly, problems such as increase in weight of the device, power consumption and manufacturing cost are unavoidable.
Second, the conventional prior art evaluates the correlation only during the guard interval period Tg. In other words, it does not evaluate the correlation during the other period (i.e., effective symbol period Tu). Accordingly, if, for example, the state of the transmission line fluctuates suddenly in the effective symbol period Tg, the fluctuation cannot be responded to immediately, and therefore instantaneous symbol loss may be caused. This drawback cannot be overlooked particularly for an OFDM receiver which may be used in a vehicle traveling at a high speed. This is because a characteristic of the transmission line significantly fluctuates during the extremely short period corresponding to the traveling speed.